- Purpose:
useEffectis a React hook used for handling side effects in functional components. Side effects can include data fetching, subscriptions, DOM manipulations, and more. - Functionality: It allows performing operations after the component renders or when certain dependencies change.
- Syntax: It takes two arguments: a function and an optional dependency array.
- Function Argument: Contains the code for the side effect.
- Dependency Array: Specifies dependencies that, when changed, will trigger the effect to run again. If this array is empty, the effect runs only once after the initial render. If omitted, the effect runs after every render.
- Dependencies: The dependencies control when the effect should be re-run. If any value in the dependency array changes between renders, the effect will execute again.
- Use Cases:
- Data Fetching: Fetching data from APIs or databases.
- Subscriptions: Setting up and cleaning subscriptions to external data sources.
- Event Listeners: Attaching and removing event listeners.
- Updating the DOM: Manipulating the DOM directly or via a library like D3.js or jQuery.
- Cleanup: The
useEffectfunction can return a cleanup function. This function executes before the effect runs again or when the component unmounts. This is useful for cleaning up resources like unsubscribing from APIs or removing event listeners. - Order of Execution: Effects are executed after every completed render, including the initial render.
- Asynchronous Operations:
useEffectcan handle asynchronous operations by usingasync/awaitor promises inside the effect function. - Usage in Next.js: In Next.js or any React application, import
useEffectfrom the ‘react’ package and use it within functional components. - Common Pitfalls:
- Incorrect dependencies might cause the effect to run more often than intended.
- Not cleaning up resources can lead to memory leaks.
- Overusing effects might make the component harder to understand.